A boron compound containing boron (10B) is likely to accumulate in cancer cells, but is unlikely to accumulate in normal cells. In this boron compound, a 10B(n,α)7Li reaction is utilized. That is, when boron (10B) captures a thermal neutron or epidermal neutron, an a particle and a lithium atom (7Li) are generated. Such an α particle and a lithium atom can selectively kill the cancer cells. This therapy has been known as boron neutron capture therapy. Conventionally, this boron neutron capture therapy has been carried out using a research reactor. However, the therapy schedule should be adjusted so as not to interfere with the operation schedule of the research reactor. Accordingly, it is not easy to make a therapy schedule. Also, problems have been caused in maintenance costs and a service life of the existing research reactor. In addition, in view of the cost and management, etc., it is markedly difficult to use a nuclear reactor as a neutron-generating device in regular hospitals.
Recently, much attention has been paid to neutron-generating devices in which protons are accelerated by an accelerator to have a predetermined energy level; and a given target material is then irradiated with the resulting protons to generate neutrons. Such neutron-generating devices are made as simple equipment when compared with nuclear reactors.
Generally speaking, target materials for these neutron-generating devices have been disclosed in Patent Literatures 1 and 2, and examples of the possible target materials include: lithium in which a 7Li(p,n)7Be reaction can be utilized; beryllium in which a 9Be(p,n) reaction can be utilized; and solid heavy metals, such as uranium, tantalum, tungsten, lead, bismuth, and mercury, in which high-energy proton-and/or deuterium-mediated nuclear spoliation reactions are utilized.
In a neutron-generating device, high-energy protons and deuterium are accelerated by an accelerator; a solid heavy metal target material is irradiated with the resulting protons and deuterium; and a nuclear spoliation reaction is used to generate high-density neutrons. Unfortunately, the accelerator of this neutron-generating device is large and expensive, and the device is therefore difficult to be installed in regular hospitals.
In addition, the neutrons generated during the nuclear spoliation reaction have markedly higher energy levels. Consequently, a large-scale neutron irradiation unit is required, including: a target having a target material; a moderate that can moderate the energy levels of the neutrons to a predetermined energy level of thermal and epidermal neutrons used for boron neutron capture therapy; and a shield that prevents the high-energy neutrons from being leaked.
Here, Patent Literature 3 discloses that the energy threshold of a proton required for the 7Li(p,n)7Be reaction is 1.889 MeV. Because of this, a proton accelerator can be small and relatively inexpensive. Thus, it has been proposed to use a metal lithium (7Li) thin film as a target material that is irradiated with accelerated protons. The metal lithium, however, is highly reactive and easily reacts with oxygen, nitrogen, and/or moisture content in the air. In view of the above, the following target structure has been disclosed (see Patent Literature 3). A process such as vapor deposition is used to form a 7Li thin film at a thickness of several dozen μm on a metal substrate. A very thin stainless steel sheet is disposed on the thin film to seal the film onto the metal substrate. Also, this target includes coolant passages through which a coolant is made to circulate to cool the metal substrate holding the metal lithium.